Transistor-based interface circuitry

ABSTRACT

Among the embodiments of the present invention is an apparatus that includes a transistor, a servo device, and a current source. The servo device is operable to provide a common base mode of operation of the transistor by maintaining an approximately constant voltage level at the transistor base. The current source is operable to provide a bias current to the transistor. A first device provides an input signal to an electrical node positioned between the emitter of the transistor and the current source. A second device receives an output signal from the collector of the transistor.

BACKGROUND

[0001] The present invention relates to electrical circuitry, and moreparticularly, but not exclusively, relates to interface circuitsincluding a transistor.

[0002] The ongoing desire for faster circuitry with fewer components hasfueled a need for better ways to interface various circuits andcircuitry components. Improved interfacing for electro-optical devices,such as photodetectors and laser generating components, is of particularinterest. Proposed interface circuits for certain photodectors typicallylimit the available frequency response and/or signal-to-noise ratio ofsuch devices. In other proposed arrangements, interfaces between certainlaser generating components and one or more corresponding input signalsources often include complicated filter networks in an attempt toprovide adequate impedance matching. Besides electro-optics, otherapplications would also benefit from better interfacing. Thus, there isa demand for further advancement in this area of technology.

SUMMARY OF INVENTION

[0003] As used herein, “transistor device” broadly refers not only to asingle transistor, but also to a transistor combined with one or moreother electronic elements to provide an active device that includes atleast three terminals. By way of nonlimiting example, transistor deviceincludes multiple transistor combinations, such as two or moretransistors connected in parallel, the Darlington configuration, and theSziklai configuration, to name a few; or different configurationsincluding at least one transistor as would occur to one skilled in theart. Further, as used herein, “transistor” broadly refers to anytransistor type, including, but not limited to, a Bipolar JunctionTransistor (BJT), Junction Field Effect Transistor (JFET), InsulatedGate Field Effect Transistor (IGFET) (where IGFETs include Metal OxideSemiconductor Field Effect Transistor (MOSFET) types). Also as usedherein, “common base” or “common gate” refers to a transistor device forwhich input and output signals of interest are each associated with atransistor device terminal other than a base or gate.

[0004] One embodiment of the present invention includes a uniqueinterface circuit. Other embodiments include unique circuits, systems,devices, apparatus, and methods for interface circuitry.

[0005] In a further embodiment, interface circuitry includes atransistor device in a common base or common gate configuration. Thisconfiguration can include a servo device that receives feedback from oneterminal of the transistor device to maintain a relatively constantlevel at that terminal.

[0006] Still a further embodiment of the present invention includes atransistor device in a common base or common gate configuration thatamplifies an input signal from a photodetector. A transistor emitter iscoupled to the photodetector to receive the input signal and an outputis provided from a transistor collector. An operational amplifier can beincluded with an output operable to drive a transistor base and anegative input coupled to the transistor emitter.

[0007] Yet another embodiment of the present invention includes:controlling operation of a transistor device in a common base or gatemode with a servo device; providing negative feedback from a firstterminal of the transistor device to a first input of the servo device;providing a selected voltage level to a second input of the servodevice; and biasing another device coupled to the first terminal inaccordance with the selected voltage level.

[0008] Another embodiment of the present invention includes: operating atransistor device in a common base or gate configuration; coupling twoor more input signal pathways to the transistor device; and providing anoutput to another device from the transistor device. In one form, thisother device is of a laser-generating type.

[0009] For another embodiment, a transistor device includes an emitter,a base, and a collector, that is arranged in a common base configurationto maintain the emitter at a predefined voltage. A number of inputsignal pathways are coupled to the emitter and a current-dependent loadis coupled to the collector. This load is responsive to an input signalreceived through one of the input signal pathways.

[0010] Circuitry of a further embodiment of the present inventionincludes a transistor device operated in a common base or gateconfiguration to provide a virtual ground at a first terminal, and alaser device electrically coupled to a second terminal of the transistordevice. Operation of the laser device is controlled with one or moreinput signals provided to the first terminal of the transistor.

[0011] Yet a further embodiment of the present invention includes:controlling a common base or common gate mode of operation of atransistor device with a servo device, where the servo device providesan output to the transistor device and receives feedback from thetransistor device; applying an approximately constant bias current tothe transistor device with a current source; receiving an input signalat an electrical node between a first terminal of the transistor deviceand the current source; and providing an output signal from thetransistor device.

[0012] One object of the present invention is to provide a uniqueinterface circuit.

[0013] Another object of the present invention is to provide a uniqueinterface circuit, system, device, apparatus, or method.

[0014] Further objects, embodiments, forms, features, advantages,benefits, and aspects of the present invention shall become apparentfrom the detailed description and drawings provided herewith.

BRIEF DESCRIPTION OF DRAWINGS

[0015]FIG. 1 is a schematic of an interface circuit of one embodiment ofthe present invention.

[0016]FIG. 2 is a schematic of circuitry of another embodiment of thepresent invention arranged to interface one or more signals with anelectrical load.

[0017]FIG. 3 is a schematic showing greater detail of one form of theembodiment shown in FIG. 2.

[0018]FIG. 4 is a schematic illustrating interface circuitry of stillanother embodiment of the present invention.

[0019]FIG. 5 is a schematic illustrating interface circuitry for aphotodiode sensor of yet another embodiment of the present invention.

DETAILED DESCRIPTION OF SELECTED EMBODIMENTS

[0020] While the present invention may be embodied in many differentforms, for the purpose of promoting an understanding of the principlesof the invention, reference will now be made to the embodimentsillustrated in the drawings and specific language will be used todescribe the same. It will nevertheless be understood that no limitationof the scope of the invention is thereby intended. Any alterations andfurther modifications in the described embodiments, and any furtherapplications of the principles of the invention as described herein arecontemplated as would normally occur to one skilled in the art to whichthe invention relates.

[0021]FIG. 1 schematically illustrates circuit 20 of one embodiment ofthe present invention. Circuit 20 includes interface circuitry 22, input(I/P) signal source 24 operable to provide an input signal to interfacecircuitry 22, and output (O/P) circuitry 26 responsive to an outputsignal from interface circuitry 22. Interface circuitry 22 changesvarious characteristics of the input signal for output to O/P circuitry26 as compared to the provision of this input signal to O/P circuitry 26directly from I/P signal source 24. Interface circuitry 22 includestransistor device 30, servo device 40, current source 50, and voltagesource 60. Signal source 24, transistor device 30, servo device 40, andcurrent source 50 are electrically coupled at a common input node 70 ofinterface circuitry 22.

[0022] Transistor device 30 is in the form of NPN bipolar junctiontransistor 31. Transistor 31 includes base 32 b electrically coupled toservo device 40, collector 32 c electrically coupled to O/P circuitry26, and emitter 32 e electrically coupled to input node 70. Servo device40 includes operational amplifier (op-amp) 41 with negative op-amp input42, positive op-amp input 44, and op-amp output 46. Negative op-ampinput 42 is electrically coupled to emitter 32 e and signal I/P source24 via input node 70. Positive op-amp input 44 is electrically coupledto voltage source 60, and op-amp output 46 is electrically coupled tobase 32 b of transistor 31.

[0023] Current source 50 is coupled to a voltage supply (−V) that isnegative relative to electrical ground. Current source 50 provides anapproximately constant current with compliance suitable to theparticular application. Current source 50 can be arranged to permit foradjustment of the output current level by an operator or otherwise, orcan be of a fixed, nonadjustable output variety.

[0024] Input signal source 24, output circuitry 26, and voltage source60 are commonly grounded. Voltage source 60 provides a voltage level topositive op-amp input 44 that is positive o relative to electricalground. Voltage source 60 provides an approximately constant voltageoutput with a degree of regulation suitable for the particularapplication. Voltage source 60 can be arranged to permit for adjustmentof the output voltage level by an operator or otherwise, or can be of afixed, nonadjustable variety.

[0025] During operation, an I/P signal from I/P signal source 24 isapplied to emitter 32 e and a corresponding O/P signal is provided toO/P circuitry 26 from collector 32 c. Transistor device 30 and servodevice 40 are configured to operate in a common base mode such that base32 b remains a generally common reference point relative to the I/Psignal at emitter 32 e and the O/P signal at collector 32 c. Operationalamplifier 41 adjust op-amp output 46, and correspondingly drives base 32b to maintain the voltage difference between negative op-amp input 42and positive op-amp input 44 close to zero. Accordingly, negative op-ampinput 42 receives negative feedback from emitter 32 e, resulting in avoltage level at emitter 32 e corresponding to that provided to positiveop-amp input 44 by voltage source 60. Furthermore, it should beunderstood that the electric current drawn by negative op-amp input 42is relatively low compared to the current flow from emitter 32 e tocollector 32 c of transistor device 30. Current source 50 provides anappropriate bias current to maintain transistor device 30 in a generallylinear conductive range for the common base configuration.

[0026] The common base mode of operation provides a way to isolatereactance characteristics of signal source 24 from O/P circuitry 26 andcorresponding provide impedance matching/transformation. Typically, theinput impedance of emitter 32 e is significantly lower than the outputimpedance of collector 32 c, which can be desirable for high frequencyinput signal conditioning and/or signal amplification, among others.When voltage source 60 provides a nonzero voltage to positive op-ampinput 44, node 70 is maintained at a comparable nonzero voltage. Thisvoltage can be used to bias certain passive forms of signal source 40,such as a sensor or detector. In another arrangement, voltage source 60can be at a zero level relative to ground, which could be alternativelyrepresented by an electrical short or resistance connection frompositive op-amp input 44 to electrical ground. For this arrangement,operational amplifier 41 acts to make the voltage difference betweenpositive op-amp input 44 and negative op-amp input 42 approach zero, sothat a virtual ground is realized at input node 70. This virtual groundarrangement can be used to provide control signals from I/P signalsource 24 to O/P circuitry 26 while isolating undesirablecharacteristics of I/P signal source 24 from O/P circuitry 26.

[0027]FIG. 2 schematically illustrates circuit 120 of another embodimentof the present invention. Circuit 120 includes interface circuit 122,input signal sources I/P1, I/P2, . . . I/Pn (collectively designated I/Pdevices 124), and output circuitry 126. Input devices 124 providesignals to output circuitry 126 via interface circuit 122. Input devices124 can provide two or more signals simultaneously, such that they aresummed together by interface circuit 122, or signals may be provided bydifferent input devices 124 at different times. While three inputdevices 124 are shown in FIG. 2, it should be understood that theellipse positioned between I/P2 and I/Pn represents the optionalpresence of more input devices 124. In still other embodiments, two orless input devices 124 could be utilized.

[0028] Interface circuit 122 includes transistor device 130 and servodevice 140 configured for a common base mode of operation. Transistordevice 130 includes PNP transistor 131 with base 132 b, collector 132 c,and emitter 132 e. Servo device 140 includes operational amplifier 141with negative op-amp input 142 coupled to input node 170 and, positiveop-amp input 144 at ground. Operational amplifier 141 also includesop-amp output 146 electrically coupled to base 132 b of transistordevice 130 via low pass (LP) filter 148. Transistor device 130 and servodevice 140 operate as described in connection with transistor device 30and servo device 40 of FIG. 1. It should be appreciated that positiveop-amp input 144 could be tied directly to ground as illustrated, orthrough a resistor as is commonly desired for many operational amplifierdevices. Likewise, low pass filter 148 may not be present, but it can bedesired for certain applications to reduce noise and provide a smootherresponse.

[0029] Input node 170 is common to emitter 132 e and input resistors R1,R2 . . . Rn. As illustrated, input node 170 is directly connected tonegative op-amp input 142; however, in other embodiments, an interfacingpassive component, such as a resistor, or network of passive componentscould be used to couple input node 170 to negative op-amp input 142.Bias current is applied to transistor device 130 by current source 150via a noise reducing low pass filter 152 coupled in series with currentsource 150. Current source 150 provides an approximately constantcurrent with a degree of compliance suitable to the particularapplication. Current source 150 can be arranged to permit adjustment ofoutput current by an operator or otherwise, or can be of anonadjustable, fixed variety.

[0030] O/P circuitry 126 includes current source 180 and load (Z) 190.Load 190 includes laser generating device 192. In one form, laser deviceis of a current-dependent load type, such as a quantum cascade laser,and current source 180 is of a variable type arranged as the maincurrent drive for load 190. In other embodiments, load 190 and device192 may be in the form of a laser diode or other laser generator, and/orinclude a different type of load.

[0031] The coupling of positive op-amp input 144 to ground provides avirtual ground at negative op-amp input 142 and correspondingly inputnode 170. Signals input to emitter 132 e from any of input devices 124are combined and output to load 190 from collector 132 c. The isolationcharacteristic of interface circuit 122 permits the combination ofsignals operating with different voltage supply rails—such thatamplifiers operating off a plus/minus five (+/−5) volt supply can becombined with those operating off a plus/minus fifteen (+/−15) voltsupply. Additionally or alternatively, high frequency modulation signalsfor load 190 can be added to other relatively slow-changing controlsignals without the need for complex interfacing filter networks.

[0032]FIG. 3 schematically illustrates circuitry 220 of anotherembodiment of the present invention. Circuitry 220 includes interfacecircuitry 222, two (2) inputs VIN1 and VIN2 (collectively designatedinputs 224) that are supplied by sources not shown, and output circuitry226. Inputs 224 can be provided by devices such as devices 124 ofcircuit 120. Interface circuit 222 includes transistor device 230 andservo device 240 arranged for a common base mode of operation aspreviously described for transistor device 30 and 130 and servo device40 and 140 for circuit 20 and 120, respectively. Interface circuit 222further includes current source circuit 250 and low pass filter circuit252 coupled in series to input node 270. Input node 270 is electricallyconnected to the emitter of transistor device 230 and coupled to thenegative input of servo device 240. The positive input of servo device240 is tied to ground via a resistor to provide a virtual ground atinput node 270 as explained in connection with circuit 120. Input VIN1is coupled to input node 270 via filter 272 and input VIN2 is coupled toinput node 270 via an input resistor.

[0033] Output circuitry 226 includes a current source circuit 280 andload output VOUT (also designated by reference numeral 290). The outputfrom the collector of transistor device 230 is provided to currentsource circuitry 280, which in turn, provides output VOUT to anelectrical load (not shown). In one form, VOUT drives a laser device ofa current-dependent variety, such as a quantum cascade type. For thisform, current source circuitry 280 is arranged to provide a desired loadcurrent that is modulated/controlled by signals from inputs 224. VIN1can be a 0-20 control voltage and VIN2 can be provided as a sweep signalfor such a form. This arrangement provides for the input of controlsignals at inputs 224, while isolating undesirable reactancecharacteristics from the load coupled to output VOUT.

[0034]FIG. 4 schematically illustrates circuitry 320 of yet anotherembodiment of the present invention. Circuitry 320 includes interfacecircuit 322; inputs 324 (individually designated as VIN1, VIN2, . . .VINn); and output circuitry 326. Inputs 324 each provide a signal tooutput circuitry 326 via interface circuit 322, and can be supplied byone or more input sources or devices (not shown). Signals from two ormore inputs 324 received simultaneously can be summed together withinterface circuit 322. While three inputs 324 are shown in FIG. 4, itshould be understood that the vertical ellipse positioned between VIN2and VINn represents the optional presence of more inputs 324. In otherembodiments, two or less inputs 324 can be utilized.

[0035] Interface circuit 322 includes PNP transistor 330 arranged for acommon base mode of operation in conjunction with PNP transistor 340.Transistor 330 includes base 332 b, collector 332 c, and emitter 332 e.Transistor 340 includes base 342 b and collector 342 c electricallycoupled together with base 332 b of transistor 330. Transistor 340 alsoincludes emitter 342 e 5 coupled to ground. Bases 332 b and 342 b, andcollector 342 c are commonly coupled to biasing current source 360.Current source 360 is arranged to provide a biasing current for theoperation of the transistor 340. The interconnection of transistors 330and 340 maintains input node 370 of interface circuitry 322 (andcorrespondingly emitter 332 e) at about the same electrical potential asemitter 342 e, except for differences that might arise due to differenttransistor sizes, collector currents, junction temperatures, and thelike. Accordingly, a virtual ground is approximated at input node 370 bythis arrangement.

[0036] Input node 370 is also coupled to input resistors R1, R2 . . . ,Rn; and a noise reducing low pass filter 352. Low pass filter 352 iscoupled in series with biasing current source 350. Current sources 350and 360 each provide an approximately constant biasing current with adegree of compliance suitable to the particular application. Currentsource 350 and/or current source 360 can be arranged to permitadjustment of output current by an operator or otherwise, or can be of anonadjustable, fixed variety.

[0037] Output circuitry 326 includes current source 380 and load (Z)390. Load 390 can be of a current-dependent type, such as a quantumcascade laser, a different laser generating arrangement, and/or adifferent load type as would occur to those skilled in the art. Thecoupling of load 390 to inputs 324 via the virtual ground provided byinterface circuit 322 provides a way to interface dissimilar signals,isolate undesirable electrical characteristics, such as reactance,and/or match/convert impedance of desired signals. In one form,circuitry 320 is applied to interface a current-dependent lasergenerating device with control/modulation signals provided from one ormore other devices via inputs 324.

[0038] Still another embodiment of the present invention isschematically illustrated as circuitry 420 in FIG. 5. Circuitry 420includes preamplifier 422 for photodetector 424 and output circuit 426.Photodetector 424 provides a signal indicative of a level of impingingphotons. In one form, photodetector 424 is of Mercury-Cadmium-Telluride(MCT) type used for long-wave infrared wavelength detection. In otherforms, photodetector 424 can be of a different type, including, but notlimited to a silicon-based sensor typically used to detect visiblelight, an Indium-Galium-Arsenide (InGaAs) type often used for nearinfrared detection, or such different type as would occur to thoseskilled in the art. Interface circuitry 422 includes transistor device430 coupled to servo device 440 to operate in the fashion previouslydescribed in connection with transistor device 30 and servo device 40 ofcircuit 20. Transistor device 430 includes PNP transistor 431 with base432 b, collector 432 c, and emitter 432 e. Servo device 440 includesoperational amplifier 441 with negative op-amp input 442, positiveop-amp input 444, and op-amp output 446. Op-amp output 446 is coupled tolow pass filter 448 to drive base 432 b of transistor 431. Emitter 432 eis coupled to input node 470 to receive input signals from photodetector424. Input node 470 is also coupled to negative op-amp input 442 toprovide negative feedback. Positive op-amp input 444 is coupled tovariable voltage source 460. Current source 450 and low pass filer 452are coupled in series to input node 470 to provide an approximatelyconstant biasing current to the transistor device 430 with a currentsource compliance suitable for the particular application. As explainedin connection with circuit 20, op-amp 441 operates to maintain voltageat node 470 at a value close to that input at positive op-amp input 444by voltage source 460. Accordingly, voltage source 460 can be adjustedto provide a desired biasing voltage level for photodetector 424.Voltage source 460 is regulated to a degree suitable for the particularapplication and, like current source 450, can be of a type that isadjustable by an operator or otherwise, or has a nonadjustable, fixedoutput.

[0039] Collector 432 c provides an amplified output of the input fromphotodetector 424 to emitter 432 e. Output circuitry 426 also includesAlternating Current (AC) amplifier 427 a. An AC voltage input to ACamplifier 427 a develops across inductor 428, while also providing forthe flow of an appropriate bias current to transistor device 430. ACamplifier 427 a of circuit 426 provides an AC output, VACOUT. Outputcircuitry 426 also includes Direct Current (DC) amplifier 427 b andsense resistor 429. The input to DC amplifier 427 b is developed acrosssense resistor 429, such that measurement of the current flow comprisedof the bias current through transistor device 430 in addition to thecurrent presented by photodetector 424 can be amplified. DC amplifier427 b of circuit 426 provides a DC output, VDCOUT. Signals VACOUT and/orVDCOUT can be further utilized by communications circuitry, sensingcircuitry, or such different applications as would occur to thoseskilled in the art.

[0040] The arrangement of circuit 420 provides a bias voltage forphotodetector 424 while at the same time isolating its reactancecharacteristics from output circuitry 426. In one form, photodetector424 is of an MCT variety and amplifier 427 a is of a high frequency,high performance 50 ohm strip line amplifier type such that performanceis comparable to the intrinsic capacitance of photodetector 424. Thisarrangement finds application in digital communications, among others.In other embodiments, a different type of photodetector and/or amplifiercould be utilized. In still other embodiments, a sensor for detectingelectromagnetic radiation other than light, a substance, and/or adifferent property/characteristic could be used instead of or inaddition to photodetector 424.

[0041] Referring generally to FIGS. 1-5, it should be appreciated thatin other embodiments, instead of a single transistor, a transistordevice that combines one or more transistors and/or one or more otheractive devices, including combinations of two or more transistors, couldbe utilized. Furthermore, it should be understood that in place of oneor more Bipolar Junction transistor (BJT) types illustrated, a FieldEffect Transistor (FET) type could be utilized. In such a case, an FETgate is used instead of the base, an FET source is used in place of oneof the collector and emitter, and an FET drain is used in place of theother of the collector and emitter. For embodiments utilizing aBJT-based transistor device in a common base configuration or operatingmode, an FET-based transistor device can be used instead of theBJT-based transistor device, with such FET-based transistor devicecorrespondingly being in a common gate configuration or operating mode.

[0042] In still other forms, a servo device utilized in accordance withthe present invention may alternatively or additionally include othertypes of circuit arrangements in addition to or as an alternative to anoperational amplifier. In one such example, a differential inputamplifier of discrete components is utilized in place of an operationalamplifier. Moreover, in yet other embodiments, different applicationsand combinations of the circuits shown in the respective embodiments areutilized.

[0043] Any theory, mechanism of operation, proof, or finding statedherein is meant to further enhance understanding of the presentinvention, and is not intended to limit the present invention in any wayto such theory, mechanism of operation, proof, or finding. While theinvention has been illustrated and described in detail in the drawingsand foregoing description, the same is to be considered as illustrativeand not restrictive in character, it being understood that only selectedembodiments have been shown and described and that all equivalents,changes, and modifications that come within the spirit of the inventionsas defined herein or by the following claims are desired to beprotected.

What is claimed is:
 1. A method, comprising: controlling a common baseor common gate mode of operation of a transistor device with a servodevice, the servo device providing an output to the transistor deviceand receiving feedback from the transistor device; applying anapproximately constant bias current to the transistor device with acurrent source; receiving an input signal at an electrical node betweena first terminal of the transistor device and the current source; andproviding an output signal from a second terminal of the transistordevice.
 2. The method of claim 1, wherein the input signal is providedby a photodiode coupled to the first terminal of the transistor device.3. The method of claim 2, further comprising biasing the photodiode inaccordance with a voltage source coupled to the servo device.
 4. Themethod of claim 1, wherein the output signal is provided to a laserdevice coupled to the collector.
 5. The method of claim 1, wherein saidcontrolling includes providing a virtual ground at the first terminal ofthe transistor device and further comprising providing a number of inputsignal pathways coupled to the first terminal of the transistor device.6. The method of claim 1, wherein the servo device includes anoperational amplifier with a negative input to receive the feedback, thefirst terminal of the transistor device corresponds to an emitter, thesecond terminal of the transistor device corresponds to a collector, andthe transistor device further includes a third terminal corresponding toa base, said third terminal receiving the output of the servo device. 7.The method of claim 1, wherein the current source is of a variable typeand further comprising adjusting the current source to change theapproximately constant bias current from a first level to a secondlevel.
 8. An apparatus, comprising: a transistor device including anemitter, a collector, and a base; a servo device operable to provide acommon base mode of operation of said transistor device; a currentsource operable to provide a bias current to said transistor device forsaid common base mode of operation; a signal source operable to providean input signal to an electrical node positioned between said emitterand said current source; and circuitry operable to receive an outputsignal from said collector.
 9. The apparatus of claim 8, wherein saidsignal source includes a photodetector and said circuitry includes anamplifier.
 10. The apparatus of claim 9, wherein said transistor device,said servo device, and said current source are arranged to provide apreamplifier for said input signal from said photodetector.
 11. Theapparatus of claim 8, wherein said servo device includes an operationalamplifier having a negative input coupled to said emitter to receivenegative feedback therefrom.
 12. The apparatus of claim 8, wherein saidcircuitry includes a laser device and said signal source includes acontrol signal generator for said laser device.
 13. The apparatus ofclaim 12, further comprising a plurality of input signal pathwayscoupled to said electrical node.
 14. The apparatus of claim 12, furthercomprising a filter positioned between said current source and saidelectrical node.
 15. A method, comprising: controlling operation of atransistor device in a common base or common gate mode with a servodevice; providing negative feedback from a first terminal of thetransistor device to a first input of the servo device; providing aselected voltage level to a second input of the servo device; andbiasing a photodetector coupled to the first terminal in accordance withthe selected voltage level.
 16. The method of claim 15, furthercomprising providing an output signal from a second terminal of thetransistor device to an amplifier.
 17. The method of claim 15, whereinthe servo device includes an operational amplifier, the operationalamplifier includes an output electrically coupled to a second terminalof the transistor device, the second terminal corresponds to a base ofthe transistor device, the first terminal corresponds to an emitter ofthe transistor device, and a third terminal of the transistor devicecorresponds to a collector.
 18. The method of claim 17, furthercomprising: providing a current source to bias the transistor device,the photodiode being coupled to the first terminal by an electrical nodebetween the current source and the first terminal; and providing afilter between the second terminal and the output of the operationalamplifier.
 19. The method of claim 18, wherein the photodetector is ofan MCT type.
 20. An apparatus, comprising: a photodetector; a transistordevice in a common base configuration operable to amplify an inputsignal from said photodetector, said transistor device including anemitter coupled to said photodetector to receive said input signal and acollector to provide a corresponding output signal; and a servo deviceincluding an output arranged to maintain a voltage level at a base ofsaid transistor device, a first input coupled to said emitter to receivenegative feedback therefrom, and a second input to receive a nonzerovoltage from a voltage source.
 21. The apparatus of claim 20, whereinsaid servo device includes an operational amplifier and saidphotodetector is of an MCT type.
 22. The apparatus of claim 20, furthercomprising a first filter coupled between said base and said output. 23.The apparatus of claim 22, further comprising: a current source to biassaid transistor device, said photodetector being coupled to said emitterby an electrical node between said current source and said emitter; anda second filter coupled between said electrical node and said currentsource.
 24. The apparatus of claim 23, wherein said current source isadjustable.
 25. The apparatus of claim 20, wherein said collectorprovides an output signal to a first amplifier.
 26. The apparatus ofclaim 25, further comprising an inductor coupled to said collector, saidfirst amplifier being coupled to a first electrical node between saidcollector and said inductor.
 27. The apparatus of claim 26, furthercomprising a resistor coupled in series with said inductor and a secondamplifier coupled across said resistor.
 28. A method, comprising:operating a transistor device in a common base or common gateconfiguration; coupling two or more input signal pathways to a firstterminal of the transistor device; and providing an output from a secondterminal of the transistor device to a current-dependent load.
 29. Themethod of claim 28, further comprising providing a virtual ground at thefirst terminal of the transistor device.
 30. The method of claim 28,further comprising: controlling the transistor device with a servodevice; and providing feedback to the servo device from the firstterminal of the transistor device.
 31. The method of claim 30, whereinthe servo device includes an operational amplifier having a negativeinput and a positive input, and further comprising: receiving thefeedback through the negative input; and coupling the positive input toground.
 32. The method of claim 28, wherein the first terminalcorresponds to an emitter, the second terminal corresponds to acollector and the transistor device further includes a base, and whichfurther includes coupling a different transistor device to the base, thedifferent transistor device including a ground-coupled emitter.
 33. Themethod of claim 28, wherein the current-dependent load includes a laserdevice, and which further includes providing a control voltage to afirst one of the input signal pathways and a sweep voltage to a secondone of the input signal pathways.
 34. The method of claim 28, whereinthe current-dependent load includes a laser device, and furthercomprising: supplying an approximately constant bias current to thetransistor device from a first current source; and providing electricalcurrent to the laser device from a second current source.
 35. Anapparatus, comprising: a transistor device including an emitter, a base,and a collector, said transistor device being in a common baseconfiguration arranged to maintain said emitter at a predefined voltage;a number of input signal pathways coupled to said emitter; and acurrent-dependent load coupled to said collector, said current-dependentload being responsive to a signal input with one of said input signalpathways.
 36. The apparatus of claim 35, wherein said current-dependentload includes a laser diode.
 37. The apparatus of claim 35, wherein saidcurrent-dependent load includes a quantum cascade laser configuration.38. The apparatus of claim 35, further comprising an operationalamplifier to control said transistor device, said operational amplifierincluding a negative input coupled to receive feedback from saidemitter, a positive input coupled to ground, and an output configured todrive said base.
 39. The apparatus of claim 35, further comprising adifferent transistor device coupled to said base, said differenttransistor device including an emitter coupled to ground.
 40. Theapparatus of claim 35, further comprising a control signal generatorcoupled to one of said inputs.
 41. The apparatus of claim 35, furthercomprising a first current source and a second current source, saidinput signal pathways being coupled to a first electrical nodepositioned between said emitter and said first current source, and saidcollector being coupled to a second electrical node positioned betweensaid second current source and said current-dependent load.
 42. Amethod, comprising: operating a transistor device in a common base orcommon gate configuration to provide a virtual ground at a firstterminal of the transistor device; electrically coupling a laser deviceto a second terminal of the transistor device; and controlling operationof the laser device with an input signal provided to the first terminalof the transistor device.
 43. The method of claim 42, which furtherincludes providing a number of input signal pathways coupled to thefirst terminal of the transistor device.
 44. The method of claim 42,wherein said operating including controlling operation of the transistordevice with a servo device, the servo device receiving feedback from thefirst terminal.
 45. The method of claim 42, further comprising:supplying an approximately constant bias current to the transistordevice from a first current source; and providing electrical current tothe laser device from a second current source.
 46. The method of claim42, wherein said operating includes regulating operation of thetransistor device with a different transistor device having an emittercoupled to ground, the transistor device and the different transistordevice having a base connection in common.